Why can't I use the field of the data class as the key of the items of the lazyColumn in jetpack compose?

A very very simple example (implementation(platform("androidx.compose:compose-bom:2023.08.00"))):

package com.study.myapplication

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.items
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview
import com.study.myapplication.ui.theme.MyApplicationTheme

data class Id(val level: Int, val index: Int)

class Man(val id: Id, val name: String)

val men = listOf(
    Man(Id(1, 1), "Amy"),
    Man(Id(1, 2), "Bob")
)

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MyApplicationTheme {
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    color = MaterialTheme.colorScheme.background
                ) {
                    Greeting(men)
                }
            }
        }
    }
}

@Composable
fun Greeting(men: List<Man>, modifier: Modifier = Modifier) {
    LazyColumn (modifier = modifier) {
        items(men, key = { it.id.index }) {   // <- render problem
        //items(men, key = { it.id }) {       // <- okay
            Text(it.name)
        }
    }
}

@Preview(showBackground = true)
@Composable
fun GreetingPreview() {
    MyApplicationTheme {
        Greeting(men)
    }
}

Refer to the comments in the code: If I use man.Id.index as key, it renders ok. If I use man.Id as key, it prompts render problem.

In a physic device, I found the real reason:

java.lang.IllegalArgumentException: Type of the key Id(level=1, index=1) is not supported. On Android you can only use types which can be stored inside the Bundle.

Why?

Solution

When you use it.id.index as the key, you're essentially using an Int value, which is a primitive type that can be easily stored in a Bundle. But, when you use it.id, you're trying to use a custom data class id, which cannot be directly stored in a Bundle because it's not a primitive type nor a type that implements Parcelable.

So If needed, you can utilize Parcelable.

@Parcelize
data class Id(val level: Int, val index: Int): Parcelable

do not forget add kotlin-parcelize plugin to build.gradle

plugins {
    id("kotlin-parcelize")
}